After performing jummah pray I uses to offer salat ul tasbeeh pray, but my grandma said to perform salat ul tasbeeh before jummah. So I just want to ask is it valid to offer salat ul tasbeeh after jummah prayer?
This prayer is considered a nafl and has no specific timing -as in the hadith quoting it there was no mentioning of a special time-. So basically you could perform it at any time of the day or night unless it is a time one shouldn't pray (non obligatory prayers) at.
Note that the word jumua'h in the hadith (which soundness is highly questionable) refers to week (not the day of jumu'ah/friday necessarily, as Arabs used to refer to the week as "jumu'ah"), so the hadith says if it's possible to pray it once a day do so, if you are able to perform it once a week do so, if you are able to perform it once a month do so, if you are able to perform it once a year do so and if not perform it at least once in lifetime. This hadith explains how this prayer should be done.
Also note that the hadith about this prayer is considered as da'if by most scholars or even fabricated by the hanbali school of fiqh and therefore they reject it:
See for example islamqa #14320 and #145112, but among the salafi scholars al-Albani has considered it as hassan.